Boron Neutron Capture Therapy: Application of Radiobiological Principles

The radiobiological characteristics of the different dose components of BNCT exposure are examined. Dose-rate determines the biological effectiveness of γ-rays, due to the capacity of cells to repair DNA damage from this low-LET radiation. Photon dose-rate has been largely overlooked in the application of BNCT. Recoil protons vary in their relative biological effectiveness (RBE) as a function of neutron energy and tissue endpoint. Thus the energy spectrum of a beam will influence the RBE of this component of the dose. Protons, of the energy produced by nitrogen capture, have not been studied. In practice protons from nitrogen capture have been combined with the recoil proton contribution into a total neutron dose. The relative biological effectiveness of the products of the boron capture reaction are a composite of the RBE of the short range products and the biodistribution of the boron, referred to collectively as the compound biological effectiveness (CBE) factor. The caution needed in the application of these factors for different normal tissues and tumors is discussed.